Advanced applications make ultrasound more competitive

Although ultrasound use in medicine continues to grow, the modality faces increasingly stiff competition from other modalities such as CT, MRI, and PET, which have undergone startling advances in the past several years. To respond to this competition, radiologists can employ several rapidly developing new technologies to enhance ultrasound's capabilities. With speckle reduction, volumetric imaging, and elastography, sonographers can reduce artifacts, improve image contrast, reduce image noise, and better gauge tissue stiffness to detect subtle hard-to-spot abnormalities. Proper use of these powerful new technologies can boost accuracy, repeatability, and efficiency to help keep ultrasound competitive with the other cross-sectional imaging modalities and perhaps open up new applications.

SPECKLE REDUCTION

Speckle, the "dots" in an ultrasound image, represents an interference pattern caused by ultrasound waves scattering off small particles in the material or tissue being scanned. These waves alternately reinforce and then cancel each other, giving rise to a pattern of dots that makes up the ultrasound image. Speckle appears because the waves in an ultrasound beam (unlike CT or MRI) are all aligned and in step with one another. This property is called "coherence." Although it allows the ultrasound beam to be tightly focused, coherence also causes the speckle that interferes with the eye's ability to detect low-contrast objects.

Technologies for reducing speckle have been tried clinically since the mid-1980s. These include image frame averaging ("persistence"), frequency compounding, spatial compounding, and use of digital filtration. The Xres feature on Philips systems and the SRI feature on GE systems are examples of filters.

Spatial compounding-commercially known as SonoCT and CrossBeam, among other names-is the process of averaging images generated from ultrasound beams traveling in different directions through the tissue. The speckle in each image is different because of the different path traversed by the beam, and the speckle tends to average out when the images are summed together. Spatial compounding can eliminate speckle if enough different images can be averaged. In practice, however, the limited number of unique directions that an ultrasound scanner can redirect the ultrasound beam from a transducer of limited size means that only a moderate reduction in speckle can be achieved by this method. Therefore, after compounding is applied, special digital filters are often used to further reduce speckle without degrading spatial resolution.

The advantages of speckle reduction include higher perceived tissue contrast, making it easier to identify areas of increased or decreased echogenicity, and decreased image noise. The decreased noise can make the image more pleasing to the eye, which might make the study more acceptable to a clinician used to the smooth appearance of CT and MR images.

Disadvantages include a reduction in frame rate, caused by the need to send and receive beams from multiple directions, a possible slight loss in spatial resolution, and loss of acoustic enhancement and shadowing, which is often important in diagnosis. Spatial compounding cannot be applied retrospectively as can digital filtration; therefore, a new acquisition must be made.

Digital filtration may also introduce artifacts that can interfere with diagnosis of certain types of lesions, but, in general, the advantages of speckle reduction vastly outweigh the disadvantages. Once introduced to speckle reduction, most sonographers and sonologists are reluctant to turn the feature completely off.

Speckle reduction enhances numerous applications, including the detection of hyperechoic or hypoechoic lesions in the kidney, liver, spleen, and many other organs (Figure 1). It is particularly effective when combined with harmonic imaging, since both can increase tissue contrast. Speckle reduction can be extremely valuable when attempting to correlate ultrasound with CT. When a lesion is suggested on a CT scan but no visible changes are noted on a conventional sonogram, for example, we turn on the SRI to a mid-to-high level to accentuate subtle borders that may be masked by speckle.

While digital filters alone can nearly eliminate speckle without degrading frame rate, using spatial compounding along with digital filtration can reduce artifacts due to filtration and can decrease shadowing from overlying tissue structures. In many cases, a low-contrast lesion seen using speckle reduction can be seen in retrospect on the original ultrasound image, barely visible due to interference from the speckle pattern.

VOLUME IMAGING

Acquisition of image data from a volume of tissue, known as volume imaging, is usually achieved by manually or automatically moving an ultrasound transducer through an area or organ perpendicular to the plane of the transducer, rather than simply sampling the volume with a small series of images. The result is a series of parallel slices that represent the volume of tissue just scanned.

We favor acquiring ultrasound data at a rate of approximately 10 frames/ sec while sweeping across an organ or area of interest at a rate of 1 cm/sec. This results in a slice spacing of approximately 1 mm and a resolution similar to the lateral resolution of the transducer. To truly achieve this high resolution, it is desirable to have the voxels be as nearly isotropic as possible. The goal is to achieve similar lateral and elevational (in the slice thickness direction) resolution. A normal ultrasound probe cannot achieve this since the beam thickness, or resolution, varies with depth. A 1.5-D array having three to five rows of crystals can dynamically focus and produce a thinner beam that varies less with depth. This type of transducer is ideal for volume imaging, and we typically use one for most of our volume scans.

Real-time ultrasound scanners have been capable of producing volume data for many years. But until the advent of high-performance PACS, it was not possible to adequately display and manipulate the data. PACS has made practical volume imaging in CT, MRI, and ultrasound possible. Typically, volume ultrasound data can be viewed by scrolling through the series of images very quickly, just as with a CT series.

In a smaller percentage of cases, manipulation of the 3D data set is needed. Manipulations include multiplanar reconstructions that generate image slices in new planes different from those actually acquired or that generate a surface rendering of a portion of the 3D data. For parenchymal organs, multiplanar reconstructions are generally the most useful (Figure 2). In obstetrics, however, surface rendering of the fetus to look for anomalies has generated the most interest.

Volume imaging has several advantages:

- Method of display on PACS is similar to CT and thus familiar to radiologists.

- Entire organ is documented, increasing diagnostic confidence that no abnormality has been missed.

- Increased confidence leads to decreased after-scanning by the radiologist, making it possible to interpret studies reliably after the patient has left the department. This dramatically improves efficiency.

- Less tedious labeling of individual images is needed, resulting in shorter examination times.

- Additional volume reconstructions can be done to increase diagnostic confidence.

- Reviewing volume sweeps at slower than real-time speed increases lesion detectability, and additional lesions not seen at real-time scanning are often detected on review of the volume data sets.

Volume imaging also faces disadvantages:

- PACS is required to properly interpret volume scans.

- A larger acoustic window is required, since an entire area or organ will be imaged in a sweep-not just a single slice.

- Volume scanning is a somewhat different skill that requires added training to ensure optimal quality.

- Some ultrasound systems cannot transfer large data sets quickly, causing delays.

In our practice, we are revising all protocols to take advantage of volume imaging. We have found that volume sweeps in two orthogonal planes give the best results. Scanning in two planes is faster than scanning in one plane and reconstructing in the other, and the additional scans may provide additional diagnostic information because of differences in acoustic windows. At the same time, acquiring too many volume scans adds to the physician time to review each one and the cost to store all the images in the PACS.

Organs that are partly obscured may be imaged using multiple short sweeps rather than a single long one covering the entire organ. Similarly, for pediatric patients, multiple short sweeps may be preferable because of patient movement.

The examiner performing sweeps for 3D acquisition must beware of the reported frame rate when using spatial compounding. Some manufacturers do not report the true frame rate in this mode. The reported frame rate must be divided by the number of views being averaged (usually three to 20) to get the true frame rate, which may be very slow. When possible, the examiner should cover the entire organ or area in a single sweep. Sweeping beyond the organ at each end ensures that the ends of the organ have been covered.

Sweeping in a direction (right to left or inferior to superior) so that the reconstructed 3D image is in the correct orientation will save time when 3D reconstructions must be performed. Certain systems with calibrated 3D, such as those by Ascension Technologies, are able to reconstruct a proper 3D volume even from sweeps acquired along a curved or irregular path. Simple linear sweeps are preferable whenever possible, however, so future examiners may reproduce the sweep with greater ease.

Careful documentation on each sweep of the orientation of the transducer and the direction of the sweep allows others to reproduce the 3D volume more easily during a future examination. Static images for measurement should be taken from the sweeps whenever possible so that the reader can place any static image within the context of a larger volume of tissue. A nodule in the upper pole of the right thyroid lobe is labeled as such, for example, but the reader can tell from the position of the image in the complete sweep how close the nodule really is to the superior edge of the thyroid.

Multiplanar reconstructions are routinely done for visualization of the endometrial cavity in pelvic ultrasound, for example. Frequently, fluid collections with complex shapes are better appreciated using 3D reconstruction, as in hydrosalpinx or some ureteral abnormalities. Other new applications for routine 3D reconstruction will surely emerge in the future.

The additional images that must be reviewed during interpretation of a volume imaging study necessitate rapid transmission of information from the sonographer to sonologist. A structured reporting form appears to be the quickest way to achieve this. Several manufacturers have rudimentary forms built into their machines that can be edited and enhanced by the user. Some PACS support structured reporting as well, and the availability of this feature and compatibility with ultrasound equipment should be carefully considered during PACS selection.

ELASTOGRAPHY TECHNIQUES

Elastography, the imaging of the elastic properties of tissue using ultrasound, has generated considerable interest and excitement in the past decade as a method of accessing new diagnostic information during sonography.

An elastogram is made by processing raw ultrasound data obtained before and after a slight compression of tissue to obtain the tissue strain at each point in the field-of-view and then displaying those strain values as an image. The strain is the change in displacement of the tissue as a function of the depth or distance from the object compressing the tissue, usually the ultrasound transducer itself.

When compressed, material in a soft object moves closer to the compressor, producing a large change in movement versus depth. A hard object such as a steel block, on the other hand, moves as a unit when compressed, producing little or no change in movement versus depth. Therefore, soft objects demonstrate large strain values in an elastogram, and hard objects demonstrate small strain values.

Sonoelastography studies the movement of tissue, usually detected with a modified color Doppler system, in response to a vibration rather than a single compression. Typically, vibrations are applied in the 100 to 500 Hz range by an acoustic transducer pressed against the tissue. Whereas elastography focuses on the elastic properties of tissue, sonoelastography produces an image related to tissue viscosity as well. A variant of sonoelastography, vibrational Doppler spectroscopy, applies vibrations at multiple different frequencies and quantifies the tissue vibratory response at each frequency. This technique can identify tissue resonant frequencies, which may be useful in distinguishing between various normal tissues and tumors.

Elastographic techniques have several advantages when used for diagnosis:

- They give information about tissue stiffness rather than backscatter intensity.

- They provide good image quality even in areas of acoustic shadowing.

- They are capable of much higher contrast-to-noise ratios than normal sonography, theoretically making abnormalities easier to detect.

Handheld elastography is feasible, and at least two manufacturers have recently incorporated elastography into their high-end clinical ultrasound units, making it more readily available for clinical testing. Sonoelastography may be performed with any color or power Doppler ultrasound system, but the user must supply an external source of vibrations.

Elastography is most easily performed on organs that are superficial and easy to compress with the ultrasound transducer. The first major application was in the breast for distinguishing benign from malignant masses.¹ This remains an important area of potential widespread clinical application, as breast cancers most often have the characteristic of being both much harder (darker) on an elastogram than other tissues (Figure 3) and larger on the elastogram than the sonogram.^1,2 The ability of elastography to detect smaller cancers and microcalcifications has not been fully studied, however.

Another area of promise is the prostate gland. By compressing the gland with the endorectal ultrasound probe, the examiner can make quality elastograms. At least one group has demonstrated the ability of elastography to detect biopsy-proven prostate cancer.³

Use of elastography for atheromatous plaque characterization is a third area of interest. Several investigators have shown the ability of elastography to accurately categorize plaque as being vulnerable to rupture and thrombosis.⁴

Finally, investigators have shown that elastography is an excellent tool for monitoring ablation therapy in the prostate and liver. Ablated lesions typically become much harder than the surrounding tissue and thus are easy to visualize and measure.⁵

New methods and parameters that may further increase the clinical value of elastography are being studied. Poisson's ratio imaging, the ratio of axial to lateral strain (Figure 4), and dynamic studies of viscoelastic and poroelastic properties may be useful in the study of interstitial fluid movement. A study of the poroelastic properties of tissues, for example, may help distinguish the amount of fibrosis versus edema in lymphedema patients and may become a useful quantitative tool for therapy monitoring.

Ultrasound evaluation of tissue elasticity has the potential to increase diagnostic accuracy and to allow the imaging of lesions that would otherwise not be visible sonographically. Clinical elastography is still in early development, but the arrival of generally available clinical systems with elastography should accelerate the maturation of this important adjunctive tool.

References

1. Garra BS, Cespedes EI, Ophir J, et al. Elastography of breast lesions: initial clinical results. Radiology 1997;202(1):79-86.

2. Hall TJ, Zhu Y, Spalding CS. In vivo real-time freehand palpation imaging. Ultrasound Med Biol 2003;29(3):427-435.

3. Konig K, Scheipers U, Pesavento A, et al. Initial experiences with real-time elastography guided biopsies of the prostate. J Urol Jul 2005;174(1):115-117.

4. Schaar JA, De Korte CL, Mastik F, et al. Characterizing vulnerable plaque features with intravascular elastography. Circulation 2003;108(21):2636-2641.

5. Varghese T, Shi H. Elastographic imaging of thermal lesions in liver in-vivo using diaphragmatic stimuli. Ultrason Imaging 2004;26(1):18-28.